Although cutting a small-diameter tube with a high degree of precision is a relatively simple affair, it is much more difficult to cut a large-diameter tube both precisely and rapidly. Rotating the tube about its center axis, lathe-fashion, while engaging it radially with one or more cutoff tools forms a very precise cut but is an unwieldly and slow process. A circular saw must have a diameter at least twice as great as that of the workpiece being cut, and can deflect somewhat at its outer rim so that the cut is not perfectly smooth and square.
Accordingly a so-called ring saw has been developed. Such a saw, as described in my French Pat. No. 2,388,625 filed Apr. 29, 1977, has an outer frame displaceable in a normally vertical plane by an appropriate actuator and carrying an inner support. A generally planar, annular and internally toothed saw blade lying generally on the plane and centered on a saw axis generally perpendicular to the plane is carried on this support. Means is provided for rotating the blade on the support about the saw axis. A drive is provided connected between the frame and the support for displacing the inner support carrying the saw so this saw orbits about the tube axis of a tube extending perpendicular to the plane through the blade. Thus when the blade is being rotated and orbited it can cut through the tube. The cut formed by such a machine is fairly good and the process is relatively rapid.
The orbiting movement in this arrangement is obtained by mounting the support on the outer ends of a plurality of parallel cranks whose inner ends are pivoted on the support. These cranks are synchronously rotated to move the support through the orbit circle. Such a crank structure is very hard to adjust for different sizes of tube, is rarely very rigid, and quickly wears so the cut produced by the saw becomes inaccurate.
In this system the frame can move to bring the saw into initial engagement and out of terminal engagement with the workpiece. Thus the system can be positioned so a new tube section can be fitted easily through it. This frame movement is pivotal about an axis parallel to the saw and normally results in the start of the cut being somewhat out of line with and not chamfered like the rest of the cut.